1. Field of the Invention
The present invention relates to multi-layered plastic molded parts which predominantly contain of aromatic polycarbonate, and which are provided with a layer of polymethacrylate plastic which contains a UV absorber at the surface, as protection against the effect of UV radiation, and a process for producing such plastic molded parts.
2. Discussion of the Background
Polycarbonate plastics have only limited weather-resistance. They are damaged by UV radiation, in particular, this damage being expressed by the fact that surfaces which were originally shiny become matte. The matte appearance is the visible manifestation of the macromolecular decomposition caused by the UV radiation, due to which a large number of extremely fine hairline cracks are formed, starting at the surface of the plastic. UV absorbers incorporated into the plastic are able to protect the interior of the plastic element, such as a plastic panel made of polycarbonate, but they cannot protect the surface. To protect the surface, a lacquer layer of polymethyl methacrylate, which also contains a UV absorber, is prepared from a solution of these protective layer components, according to U.S. Pat. No. 3,582,398.
U.S. Pat. No. 4,666,779 also describes UV-stabilized articles of thermoplastic substrates, especially those of polycarbonates which are protected by a protective layer which essentially consists of methyl methacrylate units and from 0.5 to about 10% of the number of monomer units of other methacrylate units, specifically ethyl methacrylate, n-butyl methacrylate, iso-butyl methacrylate or mixtures of these, and from 0 to 5% of the number of monomer units of other .alpha.,.beta.-unsaturated monomer units. The polymethacrylate protective layer is applied to the substrate as a solution, with the UV stabilizers having a low molecular weight, which are required for UV stabilization, being applied with the polymer-solvent system.
In addition, plastic elements are also disclosed in U.S. Pat. No. 4,576,870 and EP-A 0 203 487, in which the UV-absorbing protective layer is a copolymer essentially of (meth)acrylester monomers and monomers which contain UV protection groups. These protective layers are generally even more brittle than the protective layers which contain UV absorber and have a low molecular weight, as described above.
According to DE-A 28 32 676, a polymethyl methacrylate layer with a sufficient content of UV absorber is applied by coextrusion during production of a polycarbonate plastic panel. According to DE-A 32 44 953, which corresponds to EP-A 110 238, an improved laminate element is obtained by the fact that on top of the plastic layer which contains UV absorber, which can be polymethyl methacrylate or polycarbonate, an additional cover layer of polymethyl methacrylate or (meth)acrylate polymers, essentially containing less UV absorber, is applied during the multiple-material extrusion process.
In all cases described above, polymethacrylate plastics, particularly polymethyl methacrylate, or copolymers of methyl methacrylate and subordinate amounts of C.sub.2 to C.sub.10 alkyl or aryl (meth)acrylates esters, are used as the cover layer which contains containing UV protection, because they are highly weather-resistance and their surfaces are less easily damaged by UV radiation than those of polycarbonates--EP-B 110 221 describes polycarbonate cover layers which contain UV absorber.
The disadvantage of the known polymethacrylate cover layers for polycarbonate plastics, which contain UV absorber, is that they adhere insufficiently to the polycarbonate to be covered, and that they exhibit a lower impact resistance, in comparison with polycarbonates. These poor properties especially have a detrimental effect on the entire plastic laminate during further mechanical processing of the multi-layer plastic molded elements, such as when they are bent.